docs: redesign README and add translations

[rextio]

Summary

• reorganize the English README around project value, real proof, mechanism, first use, and visible limitations
• add or refresh Korean, Simplified Chinese, Traditional Chinese, and Japanese READMEs with matching commands and support boundaries
• add the shared Rextio icon and clear language navigation
• preserve Alpha status, fallback/fail-closed behavior, compatibility pins, and evidence qualifications

Validation

• all five localized READMEs pass the beautify-github-readme audit
• language navigation, local links, section parity, commands, versions, and warnings were checked
• the copied icon matches the supplied source asset
• git diff --check passes
• relevant existing documentation/contract tests were run where applicable

This PR changes documentation and the README icon only; it does not change product code or package configuration.